Immunophenotyping of cells and tumors, particularly hematopoietic tumors, is often of critical importance for clinical evaluation of cancer patients. However, currently available methodologies, particularly flow cytometry, are expensive and require a high degree of suspicion at the time of biopsy. All too often, even before the diagnosis of cancer is made, precious tissue must be set aside for possible immunophenotyping. If tissue is not set aside and there is cancer present, the correct subtyping of the tumor (and proper assignment to treatment protocols) cannot be done after the fact. Methods that do not require forethought, such as immunostaining of paraffin blocks, are far less sensitive and do not work well in laboratories that do not perform these stains frequently. Flow cytometry is the currently accepted “gold standard” for immunophenotyping of hematopoietic cell types. However, there are several problems with the method. The expense of establishing and maintaining these laboratories is perhaps the most severe problem. Generally large hospitals, academic centers, or commercial reference laboratories are the only institutions capable of establishing flow cytometry laboratories. These laboratories often charge a premium for their services, and transportation of specimens to laboratories is not a trivial problem. Since flow cytometry requires live cells, specimens must be handled under sterile conditions. In laboratories where the technology is unavailable, a fresh specimen has to be prepared and shipped to a flow cytometry laboratory under sterile conditions for evaluation. Uncontrollable factors such as temperature variations, rough handling, bacterial contamination, or shipping delays may render samples unsuitable for analysis. In addition, flow cytometry requires technologists who have specialized training and their time must often be dedicated solely to the technology itself, further increasing the expense of the procedure. Relatively large volumes of cells must be analyzed in order to obtain statistically meaningful results during analysis. In addition, red cells must be removed from the sample prior to analysis. This is because the number of red cells in blood and bone marrow samples is far greater than other cells types, and shear numbers alone would overwhelm the sensitive detectors of the machines. The sample preparation method therefore requires Ficoll-Hypaque separation, followed by multiple washes, followed by a lysis step to lyse remaining red cells. This method virtually eliminates megakaryocytes from most analysis and frequently destroys delicate malignant cells (particularly from the relatively common tumors such as large cell lymphoma and Hodgkin's disease). It is in these situations that the great sensitivity and complexity of flow cytometry may work to its disadvantage.
Despite the problems described above, however, flow cytometry can very accurately and with great sensitivity identify the presence of malignant cells and characterize the kind of malignant cells. Without the information that flow cytometry provides, cases can be frequently incorrectly diagnosed with catastrophic consequences for the patient. This is particularly true in the setting of a type of biopsy called fine needle aspiration where examination of a slide alone by light microscopy may be quite difficult.
What would be very useful to the average hospital pathologist or to any physician in an outpatient or remote setting is a device or kit that would allow the same kind of single parameter or multiparameter analysis of samples using cheaper, more readily available materials. This would eliminate the need for specialized laboratories and technologists dedicated solely to the flow cytometry technology itself and would allow any well trained clinical laboratorian ready access to the same kind of analysis. Furthermore, if the need for live cells could be eliminated, cells could be preserved by appropriate fixatives which would broaden the availability of immunophenotyping data.
Over the last 20 years there has been a tremendous growth in the identification and characterization of molecules expressed by blood cells on their cell membranes (called cell surface antigens). This growth in understanding has been accompanied by the refinement of technologies that allow the rapid and sensitive identification of these molecules on the surfaces of live cells. However, the overwhelming majority of these cell surface antigens are not unique to one type of cell. There is only rarely a single diagnostic marker to identify a cell type. Instead, most cell populations must be characterizing by analyzing multiple parameters at the same time.
Antibodies are proteins produced by the body's immune system that have the property that they bind to a singe specific molecule (referred to as an antigen). Antigen-antibody complexes are formed when an antibody binds its respective antigen. Normally, these complexes are then cleared by the immune system to rid the body of an infection. However, the immune system has a virtually limitless capacity to produce unique antibodies, which can be tailored to identify particular substances, even when present in very small quantities. Antibodies are now commercially produced to literally hundreds of different antigens. Furthermore, antibodies can be easily tagged with marker molecules, such as fluorescent molecules, dyes, or other substances that make identifying the presence of an antigen-antibody complex a relatively simple matter. This well-known biochemical reaction has been used to develop a methodology called flow cytometry. In flow cytometry, intact cells are treated with antibodies that bind specific markers on the cell surface. The antibodies are, in turn, labeled with a fluorescent molecule and the cell suspension then flows past a light beam with a light detector which counts the number of fluorescent cells versus the other cells present. This technology has proved tremendously useful in identifying malignant cell populations in blood and tissue samples from patients.
In flow cytometry, a cell suspension is treated with antibodies labeled with fluorescent molecules (fluorochromes), washed, and placed in the machine. The cell suspension is “focused” using buffer solutions so that the cells pass through the flow detector in a single file. When each cell passes through the flow detector, a beam of laser light is passed through the cell. Some of the light passes through the cell (called forward light scatter) and some is refracted at an angle (called side scatter). Forward scatter increases with a cell's size and side scatter increases with a cell's internal complexity (mostly granules within the cytoplasm). Thus using just these two measurements, individual cell types can be roughly categorized. However, there are also light detectors, which, by using appropriate color filters, can specifically detect the fluorescence given off by the antibodies that are attached to the cell surface. Since current state of the art machines have up to four different color detectors (referred to as four-color flow cytometry), up to four different antibodies can be added to the same tube. Samples from individual patients are usually divided into multiple tubes, each of which contains multiple antibodies. Data analysis is therefore quite complex, and requires computers that are capable of simultaneously displaying multiple plots from each tube. This is referred to as multiparameter analysis. This simultaneous analysis of multiple parameters is necessary to first electronically isolate and then characterize cell populations. Therefore, even though modern flow cytometers analyze up to 6 simultaneous parameters (forward scatter, side scatter, and four antibodies) 3 of the parameters must be used for electronic isolation of cell types (forward scatter, side scatter, and CD45 staining intensity). Broad categories of cells present in hematologic samples are known in the art and include myeloid cells, monocytes, lymphocytes, megakaryocytes, and red cells. In other words, these 3 parameters must be used to roughly mimic what the human eye does so effortlessly: identify or characterize broad categories of cells. Indeed, laboratories commonly hire technicians with 2 years of training (only part of which is in the area of hematology) who can, with a very reasonable degree of accuracy and precision, identify or characterize different cell types present in blood samples. With some additional training, they can also correctly enumerate cell types within bone marrow aspirate samples. Thus if the human eye were also equipped with the means to also identify cell surface antigens, there would be no need for flow cytometry for this purpose. Furthermore, of the remaining 3 parameters available for analysis on the flow cytometry, only 2 can be displayed in any one plot although new software exists that can display 3 dimensional plots. While 3 dimensional plots add to convenience and are applicable in limited situations, two parameter analysis is quite sufficient in most cases. This last point is critical, since any method that seeks to supplant flow cytometry must have the ability to characterize at least 2 cell surface markers simultaneously.
Analysis of cell populations by flow cytometry is not a trivial process and requires highly trained personnel as outlined above. Both single parameter and multiparameter analysis can be performed. If data is analyzed as histogram plots of fluorescence of a single marker versus cell number, then one parameter analysis is being performed. Analyzing two such histograms of a single gated cell population could then be referred to as simultaneous single parameter analysis. An example of simultaneous single parameter analysis would involve the use of such plots to identify cell surface expression of both the B-cell marker CD20 and the light chain kappa. Analysis of the binding of each set of antibodies is independent of the other. In multiparameter analysis, the binding of the two antibodies are linked and are not independent. Analytical methods require the binding of both antibodies simultaneously brought together in a single histogram such as fluorescence 1 versus fluorescence 2. Characterization of the target cell population is best performed by analysis of this fluorescence 1 vs. fluorescence 2 plot and analyzing the binding characteristics of each of these antibodies together. This decreases the possibility of an error that would incorrectly analyze two overlapping cell populations as a single cell population.
Finally, with the limited exception of DNA ploidy analysis, characterization of solid tumors and non-hematopoietic tumors is quite limited by flow cytometry. Often there are not well developed protocols for developing cell suspensions. In addition, tumor cells may be delicate and may not survive processing. In addition, many markers used for solid tumors such as vimentin or smooth muscle actin are intracytoplasmic antigens and may be difficult to assay by flow cytometry. In addition, most available markers for these other tumors are not specific markers for the tumors and many normal cells, including cells present in the background of the available sample, may be strongly positive for the same markers. Therefore, interpretation of these kinds of samples without specific morphologic correlation is hazardous at best.
An object of the invention is to provide a cheaper, more accessible method for single parameter and multiparameter analysis of cell populations. This analysis is not limited to just cell surface markers but also optionally includes identifying active receptor sites on cell surfaces, loss of cell surface proteins, intracellular proteins, and intracellular nucleic acid sequences. One of the features of this invention is that the target cell population is being analyzed by preserving morphologic characteristics of the cells for analysis. In addition, it is also possible to count events to obtain specific cell numbers in relation to specific sample volume. Due to the many preparatory steps of flow cytometry, obtaining absolute cell numbers is not possible—only percentages of cells analyzed.